Power convertor, power generation system, and power generation control method

ABSTRACT

According to one embodiment, a power convertor includes a buck-boost circuit to be applied with an input voltage to convert the input voltage into an output voltage for output, the input voltage being generated by a power generation module that generates direct-current power; a switching controller that executes maximum power-point tracking to control power conversion of the buck-boost circuit such that the power generation module generates maximum direct-current power; and a mode controller that causes the switching controller to execute the maximum power-point tracking when an input current from the power generation module is larger than a preset current threshold, and when the input current is equal to or less than the current threshold, causes the switching controller to stop the maximum power-point tracking, and cause the buck-boost circuit to output the input voltage as the output voltage without the power conversion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Applications No. 2018-209271 and No. 2018-209272, bothfiled on Nov. 6, 2018, the entire contents of all of which areincorporated herein by reference.

FIELD

Embodiments described herein relate generally to a power convertor, apower generation system, and a power generation control method.

BACKGROUND

Solar power generation systems including solar panels to generate powerare known. Solar panels exhibit a peak point on a characteristic curverepresenting generated power with respect to generated voltage. Such asolar power generation system controls power conversion to operate thesolar panels at a maximum power point, i.e., peak point. Such control isreferred to as maximum power-point tracking (MPPT) control.

To implement the MPPT control, attachment of power convertors toindividual solar panels is a known method. In this method, the powerconvertors attached to the individual solar panels are driven by thepower generated by the corresponding solar panels. Thereby, the powerconvertors can operate without receipt of power from outside.

The amount of power that can be generated by solar panels greatly variesdepending on time of day or weather. That is, in the morning or eveningor due to cloudy weather for example, the solar panels may be able togenerate only a small amount of power. In such a case the powerconvertor consumes a larger amount of power than an increased amount ofpower of the solar panels under the MPPT control. Thus, the powerconvertors cannot efficiently output the power generated by the solarpanels.

It is useful to provide a power converter, a power generation system,and a power generation control method which enable efficient poweroutput while power generation modules cannot generate sufficient amountof power.

SUMMARY

A power convertor according to this disclosure includes a buck-boostcircuit, a switching controller, and a mode controller. The buck-boostcircuit is to be applied with an input voltage to convert the inputvoltage into an output voltage for output. The input voltage isgenerated by a power generation module that generates direct-currentpower. The switching controller executes maximum power-point tracking tocontrol power conversion of the buck-boost circuit such that the powergeneration module generates maximum direct-current power. The modecontroller causes the switching controller to execute the maximumpower-point tracking when an input current from the power generationmodule is larger than a preset current threshold; and cause theswitching controller to stop the maximum power-point tracking, and causethe buck-boost circuit to output the input voltage as the output voltagewithout the power conversion when the input current is equal to or lessthan the current threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary configuration of a powergeneration system;

FIG. 2 is a diagram illustrating power generation efficiency with orwithout power convertors;

FIG. 3 is a diagram illustrating an exemplary configuration of the powerconvertor;

FIG. 4 is a diagram illustrating an exemplary functional configurationof a controller according to a first embodiment;

FIG. 5 is a diagram illustrating the states of switches of a buck-boostcircuit in a stop mode;

FIG. 6 is a diagram illustrating the states of the switches in thebuck-boost circuit in a pass-through mode;

FIG. 7 is a diagram illustrating the states of the switches in thebuck-boost circuit during voltage step-down in a tracking mode;

FIG. 8 is a diagram illustrating a waveform of a first switching signalduring voltage step-down in the tracking mode;

FIG. 9 is a diagram illustrating the states of the switches in thebuck-boost circuit during voltage step-up in the tracking mode;

FIG. 10 is a diagram illustrating a waveform of a third switching signalduring voltage step-up in the tracking mode;

FIG. 11 is a diagram for illustrating a variation in input voltage byhill climbing;

FIG. 12 is a flowchart of processing by the hill climbing;

FIG. 13 is a diagram illustrating operation modes of the power convertoraccording to the first embodiment;

FIG. 14 is a state transition diagram of the operation modes in thefirst embodiment;

FIG. 15 is a diagram illustrating an exemplary functional configurationof a controller according to a second embodiment;

FIG. 16 is a state transition diagram of operation modes in the secondembodiment;

FIG. 17 is a state transition diagram of operation modes in amodification of the second embodiment;

FIG. 18 is a diagram for illustrating a variation in input voltageduring scanning in a third embodiment;

FIG. 19 is a diagram illustrating the states of switches in thebuck-boost circuit when a value of an input voltage from the solar panelvaries in a given range;

FIG. 20 is a flowchart of the processing of a switching controlleraccording to the third embodiment;

FIG. 21 is a diagram illustrating variations in input voltage and inputcurrent in the processing in FIG. 20;

FIG. 22 is a diagram illustrating an exemplary functional configurationof a controller according to a fourth embodiment;

FIG. 23 is a state transition diagram of a switching controlleraccording to the fourth embodiment; and

FIG. 24 is a diagram illustrating an exemplary variation in acharacteristic curve representing generated power with respect togenerated voltage at the time of occurrence of an efficiency changingevent.

DETAILED DESCRIPTION

A plurality of embodiments will be described below. Elements common tothe embodiments are denoted by the same reference numerals.Configurations or structures common to the embodiments are described indetail in a first embodiment, and detailed descriptions thereof areomitted in the subsequent embodiments except for differences.

First Embodiment

FIG. 1 is a diagram illustrating a power generation system 10 accordingto a first embodiment by way of example. The power generation system 10includes a plurality of solar panels 20, a combiner device 22, a powerconditioner 24, and one or two or more power convertors 30.

The solar panels 20 receive solar light, and convert the received solarlight into electric energy. The solar panels 20 generate DC power.

The solar panels 20 each include a plurality of clusters. For example,each solar panel 20 includes two or three clusters. Each clusterincludes a plurality of solar cells or power generation cells connectedin series. The clusters of the solar panels 20 are connected in series.While all of the clusters of the individual solar panels 20 normallyreceive solar light to generate power, bypass diodes connected inparallel to all of the clusters are turned off, receiving no bypasscurrent. As a result, as long as all of the clusters in the solar panel20 normally receive solar light to generate power, the solar panel 20generally generates power (in general efficiency state). If part of theclusters of the solar panel 20 is shadowed or fails to operate properly,however, the bypass diode parallel-connected to the partly shaded orfaulty cluster is turned on, receiving a bypass current. That is, withpart of the clusters shaded or having failed, the solar panel 20generates power at lower efficiency than usual (in low efficiencystate).

The power generation system 10 includes one or two or more strings. Onestring includes a plurality of solar panels 20. The solar panels 20 inone string are connected in series.

The combiner device 22 connects DC power output from the strings inparallel, and supplies the DC power to the power conditioner 24. Thecombiner device 22 prevents the current from reversely flowing from astring to another string. For prevention of the reverse flow, eachstring is connected forward to a diode at a terminal that generates apositive voltage.

The power conditioner 24 receives and converts DC power from thecombiner device 22 into AC power with a given frequency. The powerconditioner 24 outputs the generated AC power to outside through a powerline. The power conditioner 24 serves to protect interconnected systems.

The power conditioner 24 executes maximum power-point tracking (MPPT)control over the entire solar panels 20. The solar panels 20 exhibit apeak point or a local maximum point on a characteristic curverepresenting generated power with respect to generated voltage. Throughthe maximum power-point tracking control, the power conditioner 24allows the solar panels 20 to operate at the maximum operating point.

All or part of the solar panels 20 have power convertors 30 attachedthereto. The power convertors 30 receive and convert DC power from thesolar panels 20 into DC power (DC to DC conversion). The powerconvertors 30 of the solar panels 20 are connected in series at outputend to another solar panel 20.

FIG. 1 illustrates an example that all of the solar panels 20 have thepower convertors 30 attached thereto. However, the power generationsystem 10 may include solar panel or panels 20 with no power convertor30 attached.

The power generation system 10 according to the first embodiment mayinclude, instead of the solar panels 20, another power generation modulethat exhibits a peak point on the characteristic curve representinggenerated power with respect to generated voltage. For example, thepower generation module may be a wind-power generator or fuel cells.

FIG. 2 is a diagram illustrating power generation efficiency with orwithout the power convertors 30. The power convertors 30 execute maximumpower-point tracking control over the corresponding solar panels 20 forpower conversion to operate them at a maximum power point.

For example, with no power convertors 30 attached to the solar panels20, along with a decrease in power generation efficiency of one solarpanel 20, the other solar panels 20 of the same string will decrease inpower generation efficiency.

However, in the case of the solar panel 20 with the power convertor 30,the power convertor 30 can step up or down voltage generated by thecorresponding solar panel 20 to supply the voltage to the string tooperate the corresponding solar panel 20 at a maximum power point,irrespective of a decrease in power generation efficiency of the solarpanel 20. Thus, the other solar panels 20 in the same string can alsooperate at the maximum power point. That is, irrespective of a decreasein power generation efficiency of the corresponding solar panel 20, thepower convertor 30 can minimize the decrease in power generationefficiency of the other solar panels 20 of the string.

FIG. 3 is a diagram illustrating an exemplary configuration of the powerconvertor 30. The power convertor 30 includes a positive input terminal42, a negative input terminal 44, a positive output terminal 46, anegative output terminal 48, a buck-boost circuit 50, an ammeter 52, aninput-side voltmeter 54, an output-side voltmeter 56, a controller 60,and a power supply 62.

The positive input terminal 42 and the negative input terminal 44 areapplied with an input voltage generated from the corresponding solarpanel 20. The positive input terminal 42 is connected to a positiveterminal of the corresponding solar panel 20. The negative inputterminal 44 is connected to a negative terminal of the correspondingsolar panel 20.

The positive output terminal 46 is connected to a negative terminal ofanother solar panel 20 adjacent to the positive side in the same stringor the negative output terminal 48 of the power convertor 30 of anotheradjacent solar panel 20. The positive output terminal 46 of the solarpanel 20 located at the positive-side end of the string is connected tothe combiner device 22.

The negative output terminal 48 is connected to a positive terminal ofanother solar panel 20 adjacent to the negative side in the same stringor the positive output terminal 46 of the power convertor 30 of anothersolar panel 20 adjacent to the negative side. The negative outputterminal 48 of the solar panel 20 located at the negative-side end ofthe string is connected to the combiner device 22.

The ammeter 52 measures a value of current (input current I_(IN)) fromthe corresponding solar panel 20. In the present embodiment, the ammeter52 measures current flowing from the positive input terminal 42 to thepositive output terminal 46. For example, the ammeter 52 includes acurrent-measurement resistor with a minute resistance value, inserted ina path between the positive input terminal 42 and the positive outputterminal 46, and an amplifier for amplifying a voltage generated in thecurrent measurement resistor. Such an ammeter 52 outputs to thecontroller 60 voltage of a value proportional to the current (inputcurrent I_(IN)) from the corresponding solar panel 20.

The input-side voltmeter 54 measures a value of voltage (input voltageV_(IN)) generated from the corresponding solar panel 20. In the presentembodiment, the input-side voltmeter 54 measures a voltage in-betweenthe positive input terminal 42 and the negative input terminal 44. Forexample, the input-side voltmeter 54 includes an input-voltage detectionresistor with a large resistance value, located between the positiveinput terminal 42 and the negative input terminal 44, and an amplifierfor amplifying a voltage generated in the input-voltage detectionresistor. Such an input-side voltmeter 54 outputs to the controller 60voltage of a value proportional to the voltage (input voltage V_(IN))generated from the corresponding solar panel 20.

The output-side voltmeter 56 measures a value of a voltage (outputvoltage V_(OUT)) output from the buck-boost circuit 50. In the presentembodiment, the output-side voltmeter 56 measures a voltage in-betweenthe positive output terminal 46 and the negative output terminal 48. Forexample, the output-side voltmeter 56 includes an output-voltagedetection resistor with a large resistance value, located between thepositive output terminal 46 and the negative output terminal 48, and anamplifier for amplifying a voltage generated in the output voltagedetection resistor. Such an output-side voltmeter 56 outputs to thecontroller 60 a voltage of a value proportional to the voltage (outputvoltage V_(OUT)) from the buck-boost circuit 50.

The buck-boost circuit 50 is applied with an input voltage V_(IN) of DCpower generated from the corresponding solar panel 20. The buck-boostcircuit 50 outputs the output voltage V_(OUT) of DC powerpower-converted from the input voltage V_(IN). The buck-boost circuit 50represents an H-bridge chopper circuit. The buck-boost circuit 50 canstep down the input voltage V_(IN) to an output voltage V_(OUT)(V_(IN)>V_(OUT)), and step up the input voltage V_(IN) (V_(IN)<V_(OUT))to an output voltage V_(OUT) for output. The buck-boost circuit 50 candirectly output the input voltage V_(IN) as the output voltage V_(OUT)without power conversion (V_(IN)=V_(OUT)).

In the present embodiment, the buck-boost circuit 50 includes aninductor 70, a first switch 72, a second switch 74, a third switch 76, afourth switch 78, and a capacitor 80.

The first switch 72 switches on and off a path between the positiveinput terminal 42 and a first terminal 70-1 of the inductor 70 under thecontrol of the controller 60. The second switch 74 switches on and off apath between the negative input terminal 44 and the first terminal 70-1of the inductor 70 under the control of the controller 60. The thirdswitch 76 switches on and off a path between a second terminal 70-2 ofthe inductor 70 and the positive output terminal 46 under the control ofthe controller 60. The fourth switch 78 switches on and off a pathbetween the second terminal 70-2 of the inductor 70 and the negativeoutput terminal 48 under the control of the controller 60. The capacitor80 is connected in-between the positive output terminal 46 and thenegative output terminal 48.

The first switch 72 represents, for example, an re-channel metal oxidesemiconductor field effect transistor (MOSFET). The first switch 72 hasa drain connected to the positive input terminal 42, a source connectedto the first terminal 70-1 of the inductor 70, and a gate supplied witha first switching signal S₁ from the controller 60.

The second switch 74 represents, for example, an re-channel MOSFET. Thesecond switch 74 has a drain connected to the first terminal 70-1 of theinductor 70, a source connected to the negative input terminal 44, and agate supplied with a second switching signal S₂ from the controller 60.

The third switch 76 represents, for example, an re-channel MOSFET. Thethird switch 76 has a source connected to the second terminal 70-2 ofthe inductor 70, a drain connected to the positive output terminal 46,and a gate supplied with a third switching signal S₃ from the controller60.

The fourth switch 78 represents, for example, an re-channel MOSFET. Thefourth switch 78 has a drain connected to the second terminal 70-2 ofthe inductor 70, a source connected to the negative output terminal 48,and a gate supplied with a fourth switching signal S₄ from thecontroller 60.

The controller 60 represents a microcomputer, and controls the operationof the buck-boost circuit 50. The power supply 62 receives DC power fromthe corresponding solar panel 20 through the positive input terminal 42and the negative input terminal 44, and stabilizes a DC voltage foroutput. The power supply 62 applies the stabilized DC voltage to thecontroller 60. Thus, the controller 60 is driven by the DC powergenerated by the corresponding solar panel 20.

In the present embodiment, the controller 60 includes a centralprocessing unit (CPU) 82, a read only memory (ROM) 84, a random accessmemory (RAM) 86, an analog-to-digital converter (ADC) 88, and an I/Fcircuit 90. These elements are connected via a bus.

The CPU 82 executes various kinds of processing in coordination withvarious kinds of computer programs pre-stored in the ROM 84, using agiven area of the RAM 86 as a work area, and comprehensively controlsthe operations of the elements of the controller 60. The CPU 82 operatesthe ADC 88 and the I/F circuit 90 in coordination with computer programspre-stored in the ROM 84.

The ROM 84 stores in a non-rewritable manner computer programs andvarious kinds of setup information used in the control by the controller60. The RAM 86 represents a volatile storage medium, such as a dynamicrandom access memory (DRAM). The RAM 86 functions as a work area for theCPU 82.

The ADC 88 converts voltages output from the ammeter 52, the input-sidevoltmeter 54, and the output-side voltmeter 56 into digital values.Thereby, the CPU 82 can acquire the value of the input current I_(IN),the value of the input voltage V_(IN), and the value of the outputvoltage V_(OUT).

The I/F circuit 90 outputs the first switching signal S₁, the secondswitching signal S₂, the third switching signal S₃, and the fourthswitching signal S₄ under the control of the CPU 82. Thereby, thebuck-boost circuit 50 can operate under the control of the controller60.

FIG. 4 is a diagram illustrating an exemplary functional configurationof the controller 60 according to the first embodiment. The controller60 functions as the elements illustrated in FIG. 4 by a computer programstored in the ROM 84 and executed by the CPU 82.

Specifically, the controller 60 includes a current-value acquirer 102, avoltage-value acquirer 104, a power calculator 106, a switch driver 108,a switching controller 110, and a mode controller 112.

The current-value acquirer 102 acquires a value of the input currentI_(IN) from the corresponding solar panel 20, the value measured by theammeter 52. The voltage-value acquirer 104 acquires a value of the inputvoltage V_(IN) generated from the corresponding solar panel 20 andmeasured by the input-side voltmeter 54. The voltage-value acquirer 104further acquires a value of the output voltage V_(OUT) from thebuck-boost circuit 50, the value measured by the output-side voltmeter56. The current-value acquirer 102 and the voltage-value acquirer 104are implemented by the ADC 88 and the CPU 82.

The power calculator 106 calculates DC power generated by thecorresponding solar panel 20 from the value of the input voltage V_(IN)and the value of the input current I_(IN). The power calculator 106 isimplemented by the CPU 82.

The switch driver 108 outputs the first switching signal S₁, the secondswitching signal S₂, the third switching signal S₃, and the fourthswitching signal S₄ to drive the first switch 72, the second switch 74,the third switch 76, and the fourth switch 78, respectively. The switchdriver 108 is implemented by the I/F circuit 90 and the CPU 82.

In response to a command for designating the operation mode from themode controller 112, the switch driver 108 turns on or off the firstswitch 72, the second switch 74, the third switch 76, and the fourthswitch 78.

In response to receipt of a tracking mode command for the operationmode, the switch driver 108 performs switching of the first switch 72,the second switch 74, the third switch 76, and the fourth switch 78under the control of the switching controller 110. Thereby, in responseto receipt of a tracking mode command for the operation mode, the switchdriver 108 can step down or up the input voltage V_(IN) to output anoutput voltage V_(OUT) to the buck-boost circuit 50.

In response to receipt of a stop mode command for the operation mode,the switch driver 108 turns off the first switch 72, the second switch74, the third switch 76, and the fourth switch 78. Thus, in response toreceipt of the stop mode command for the operation mode, the switchdriver 108 can open the path between the positive output terminal 46 andthe negative output terminal 48 to allow the buck-boost circuit 50 tostop outputting the output voltage V_(OUT).

In response to receipt of a pass-through mode command for the operationmode, the switch driver 108 turns on the first switch 72 and the thirdswitch 76 and turns off the second switch 74 and the fourth switch 78.That is, in response to receipt of the pass-through mode for theoperation mode, the switch driver 108 can connect in-between thepositive input terminal 42 and the positive output terminal 46 in a DCmanner and connect between the negative input terminal 44 and thenegative output terminal 48 in a DC manner to allow the buck-boostcircuit 50 to output the input voltage V_(IN) directly as the outputvoltage V_(OUT) without power conversion.

The switching controller 110 executes maximum power-point tracking tocontrol the power conversion of the buck-boost circuit 50 such that thecorresponding solar panel 20 generates maximum DC power. The switchingcontroller 110 is implemented by the CPU 82.

In the maximum power-point tracking, the switching controller 110acquires a target conversion ratio representing a target ratio of avalue of the output voltage V_(OUT) to a value of the input voltageV_(IN). The switching controller 110 controls the switching of the firstswitch 72, the second switch 74, the third switch 76, and the fourthswitch 78 such that the ratio between the value of the input voltageV_(IN) and the value of the output voltage V_(OUT) turns to the targetconversion ratio.

In the maximum power-point tracking, the switching controller 110 causesthe buck-boost circuit 50 to switch between a step-down operation and astep-up operation in accordance with the target conversion ratio. Thetarget conversion ratio is expressed in percentage, for example. In thiscase, the switching controller 110 controls the buck-boost circuit 50 toperform the step-down operation when the target conversion ratio issmaller than 100%, and controls the buck-boost circuit 50 to perform thestep-up operation when the target conversion ratio exceeds 100%.

The switching controller 110 executes hill climbing as an example of themaximum power-point tracking. For another example, the switchingcontroller 110 may execute scanning as the maximum power-point tracking.

The mode controller 112 controls the operation mode of the powerconvertor 30 in accordance with the input voltage V_(IN) and the inputcurrent I_(IN). The mode controller 112 is implemented by the CPU 82.

More specifically, with the input current I_(IN) from the correspondingsolar panel 20 exceeding a preset current threshold I_(T), the modecontroller 112 sets the operation mode of the power convertor 30 to atracking mode. In the tracking mode, the mode controller 112 controlsthe switching controller 110 to execute maximum power-point tracking.

With the input current I_(IN) being the current threshold I_(T) or less,the mode controller 112 sets the operation mode of the power convertor30 to a pass-through mode. In the pass-through mode, the mode controller112 controls the switching controller 110 to stop the maximumpower-point tracking. In the pass-through mode, the mode controller 112instructs the switch driver 108 to control the buck-boost circuit 50 tooutput the input voltage V_(IN) directly as the output voltage V_(OUT)without power conversion.

When the input voltage V_(IN) is equal to or less than a preset voltagethreshold V_(T), the mode controller 112 sets the operation mode of thepower convertor 30 to a stop mode irrespective of the input currentI_(IN). In the stop mode, the mode controller 112 controls the switchingcontroller 110 to stop the maximum power-point tracking. In the stopmode, the mode controller 112 instructs the switch driver 108 to controlthe buck-boost circuit 50 to stop outputting the output voltage V_(OUT).

FIG. 5 is a diagram illustrating the states of the switches of thebuck-boost circuit 50 in the stop mode. In response to the stop modecommand from the mode controller 112 as the operation mode, the switchdriver 108 turns off the first switch 72, turns off the second switch74, turns off the third switch 76, and turns off the fourth switch 78.

Thereby, in the stop mode, the switch driver 108 can open the pathbetween the positive output terminal 46 and the negative output terminal48. In the stop mode, the switch driver 108 can open the path betweenthe corresponding solar panel 20 and the inductor 70. In this manner,the switch driver 108 can allow the buck-boost circuit 50 to stop thepower conversion, and prevent the buck-boost circuit 50 from outputtingthe output voltage V_(OUT).

FIG. 6 is a diagram illustrating the states of the switches of thebuck-boost circuit 50 in the pass-through mode. In response to apass-through mode command from the mode controller 112 as the operationmode, the switch driver 108 turns on the first switch 72, turns off thesecond switch 74, turns on the third switch 76, and turns off the fourthswitch 78.

The inductor 70 is equivalent to wiring having a resistance value ofzero in terms of direct current. Thus, in the pass-through mode, theswitch driver 108 can connect in-between the positive input terminal 42and the positive output terminal 46, and connect in-between the negativeinput terminal 44 and the negative output terminal 48. Thereby, in thepass-through mode, the switch driver 108 can control the buck-boostcircuit 50 to output the input voltage V_(IN) directly as the outputvoltage V_(OUT) without power conversion.

FIG. 7 is a diagram illustrating the states of the switches of thebuck-boost circuit 50 during voltage step-down in the tracking mode.

In the tracking mode, the switching controller 110 changes the switchingmethod of the buck-boost circuit 50 during voltage step-down when thetarget conversion ratio is smaller than 100% and during voltage step-upwhen the target conversion ratio is larger than 100%.

During the voltage step-down in the tracking mode, the switchingcontroller 110 performs switching as illustrated in FIG. 7.Specifically, the switching controller 110 fixedly turns on the thirdswitch 76 and off the fourth switch 78. The switching controller 110complementally switches on and off the first switch 72 and the secondswitch 74 in a given switching cycle.

Complementally switching on and off the first switch 72 and the secondswitch 74 refers to turning off the second switch 74 while the firstswitch 72 is ON, and turning on the second switch 74 while the firstswitch 72 is OFF.

FIG. 8 is a diagram illustrating the first switching signal S₁ duringvoltage step-down in the tracking mode. During the voltage step-down inthe tracking mode at the target conversion ratio being less than 100%,the switching controller 110 sets an on-period of the first switch 72such that the larger the target conversion ratio is, the longer theon-period is.

At the target conversion ratio being less than 100%, for example, theswitching controller 110 complementally switches on and off the firstswitch 72 and the second switch 74 such that the on-period of the firstswitch 72 with respect to the switching cycle is set to a proportioncorresponding to the target conversion ratio. More specifically, theswitching controller 110 sets the on-period T_(1ON) and the off periodT_(1OFF) of the first switch 72 to the values defined by the followingequations:T _(10N) =T×R/100T _(1OFF) =T−T _(1ON)where T represents the given switching cycle and R represents the targetconversion ratio expressed in percentage.

In this manner, during the voltage step-down, the switching controller110 can control the buck-boost circuit 50 to output an output voltageV_(OUT) of a value obtained by multiplying the value of the inputvoltage V_(IN) by the target conversion ratio.

FIG. 9 is a diagram illustrating the states of the switches of thebuck-boost circuit 50 during voltage step-up. During voltage step-up inthe tracking mode, the switching controller 110 performs switching asillustrated in FIG. 9.

Specifically, the switching controller 110 fixedly turns on the firstswitch 72 and off the second switch 74. The switching controller 110complementally switches on and off the third switch 76 and the fourthswitch 78 in a given switching cycle. Complementally switching on andoff the third switch 76 and the fourth switch 78 refers to turning offthe fourth switch 78 while the third switch 76 is ON, and turning on thefourth switch 78 while the third switch 76 is OFF.

FIG. 10 is a diagram illustrating the third switching signal S₃ duringvoltage step-up. During the voltage step-up in the tracking mode at thetarget conversion ratio exceeding 100%, the switching controller 110sets the on-period of the third switch 76 such that the larger thetarget conversion ratio is, the longer as the on-period is.

At the target conversion ratio exceeding 100%, for example, theswitching controller 110 complementally switches one and off the thirdswitch 76 and the fourth switch 78 such that the on-period of the thirdswitch 76 with respect to the switching cycle is set to a proportioncorresponding to a value found by dividing, by the target conversionratio, a value being a resultant of subtracting 100% from the targetconversion ratio. More specifically, for example, the switchingcontroller 110 sets the on-period T_(3ON) and the off period T_(3OFF) ofthe third switch 76 to the values defined by the following equations:T _(30N) =T×(R−100)/RT _(3OFF) =T−T _(30N)where T represents the given switching cycle and R represents the targetconversion ratio expressed in percentage.

In this manner, during voltage step-up, the switching controller 110 cancontrol the buck-boost circuit 50 to output the output voltage V_(OUT)of a value obtained by multiplying the value of the input voltage V_(IN)by the target conversion ratio.

FIG. 11 illustrates variation in the input voltage V_(IN) in hillclimbing. The hill climbing refers to a maximum power-point trackingcontrol method for the buck-boost circuit 50 while continuouslyperforming power conversion, i.e., without suspending power conversion.

By hill climbing, the switching controller 110 minutely increases ordecreases the target conversion ratio while allowing the buck-boostcircuit 50 to continue power conversion. The switching controller 110monitors a variation in power generated by the solar panel 20 to changethe target conversion ratio such that the solar panel 20 generatesincreased power. As a result, the switching controller 110 can increaseor decrease the target conversion ratio such that the value of the inputvoltage V_(IN) reciprocates across a peak power point. Thereby, theswitching controller 110 can operate the corresponding solar panel 20 ata maximum power point.

FIG. 12 is a flowchart illustrating the procedure of the hill climbing.For example, the switching controller 110 executes the processing forthe hill climbing, as illustrated in FIG. 12.

The switching controller 110 repeats the processing from S212 to S219,i.e., loop operation between S211 and S220) in each given time. At S212,the switching controller 110 acquires a value of DC power generated bythe corresponding solar panel 20.

Subsequently, at S213, the switching controller 110 determines whetherthe target conversion ratio has been increased in the previous loopoperation. With an increase in the target conversion ratio (Yes atS213), the switching controller 110 proceeds to S214. With no increasein the target conversion ratio, that is, decrease in the targetconversion ratio (No at S213), the switching controller 110 proceeds toS215.

At S214, the switching controller 110 compares a value of DC powercalculated in the previous loop operation with a value of DC powercalculated in the current loop operation. With an increase in the power(Yes at S214), the switching controller 110 proceeds to S216. With noincrease in the power (No at S214), the switching controller 110proceeds to S217.

At S216, the switching controller 110 increases the target conversionratio by a given amount. At S217, the switching controller 110 decreasesthe target conversion ratio by a given amount.

At S215, the switching controller 110 compares a value of DC powercalculated in the previous loop operation with a value of DC powercalculated in the current loop operation. With an increase in the power(Yes at S215), the switching controller 110 proceeds to S218. With noincrease in the power (No at S215), the switching controller 110proceeds to S219.

At S218, the switching controller 110 decreases the target conversionratio by a given amount. At S219, the switching controller 110 increasesthe target conversion ratio by a given amount.

After completing the processing at S216, S217, S218, or S219, theswitching controller 110 repeats the processing from S212, i.e., loopoperation between S211 and S220 after a given length of time.

As described above, if the DC power has increased by increasing thetarget conversion ratio, the switching controller 110 further increasesthe target conversion ratio, and if the DC power has decreased byincreasing the target conversion ratio, the switching controller 110decreases the target conversion ratio. If the DC power has increased bydecreasing the target conversion ratio, the switching controller 110further decreases the target conversion ratio, and if the DC power hasdecreased by further decreasing the target conversion ratio, theswitching controller 110 increases the target conversion ratio.

The switching controller 110 repeats such processing in each given time.Thereby, the switching controller 110 can operate the solar panel 20 ata maximum power point.

FIG. 13 is a diagram illustrating the operation modes of the powerconvertor 30 according to the first embodiment. The power convertor 30according to the first embodiment operates in three operation modes: astop mode, a tracking mode, and a pass-through mode.

The mode controller 112 is provided with a preset current thresholdI_(T) and a preset voltage threshold V_(T). The mode controller 112compares an input current I_(IN) and an input voltage V_(IN) from thesolar panel 20 with the current threshold I_(T) and the voltagethreshold V_(T) to switch the operation modes.

FIG. 14 is a state transition diagram of the operation modes in thefirst embodiment. At start of power generation by the correspondingsolar panel 20, the mode controller 112 sets the power convertor 30 inthe stop mode.

In the stop mode, the mode controller 112 compares the input voltageV_(IN) with the voltage threshold V_(T). At the input voltage V_(IN)being equal to or less than the voltage threshold V_(T) in the stopmode, the mode controller 112 maintains the stop mode of the powerconvertor 30.

In the stop mode, with the input voltage V_(IN) exceeding the voltagethreshold V_(T) and the input current I_(IN) being the current thresholdI_(T) or less, the mode controller 112 controls the power convertor 30to transition to the pass-through mode. In the stop mode, with the inputvoltage V_(IN) exceeding the voltage threshold V_(T) and the inputcurrent I_(IN) exceeding the current threshold I_(T), the modecontroller 112 controls the power convertor 30 to transition to thetracking mode.

In the pass-through mode, at the input voltage V_(IN) decreasing to thevoltage threshold V_(T) or less, the mode controller 112 controls thepower convertor 30 to transition to the stop mode. In the pass-throughmode, with the input voltage V_(IN) exceeding the voltage thresholdV_(T) and the input current I_(IN) exceeding the current thresholdI_(T), the mode controller 112 controls the power convertor 30 totransition to the tracking mode.

In the tracking mode, at the input voltage V_(IN) being the voltagethreshold V_(T) or less, the mode controller 112 controls the powerconvertor 30 to transition to the stop mode. In the tracking mode, withthe input voltage V_(IN) exceeding the voltage threshold V_(T) and theinput current I_(IN) being the current threshold I_(T) or less, the modecontroller 112 controls the power convertor 30 to transition to thepass-through mode.

The amount of power that can be generated by the solar panel 20 greatlyvaries depending on time of day and weather. Thus, in the morning orevening or due to cloudy weather, for example, the solar panel 20 cangenerate only small amount of power. In such a situation the switchdriver 108 consumes a larger amount of power for driving the switchesthan increased amount of generated power through the maximum power-pointtracking. At the time of the input current I_(IN) decreasing to thepreset current threshold I_(T) or less, the power convertor 30 accordingto the present embodiment sets the buck-boost circuit 50 in thepass-through mode to stop switching. In this manner, with the inputcurrent I_(IN) being the preset current threshold I_(T) or less, thepower convertor 30 according to the present embodiment can prevent theswitch driver 108 from consuming power for driving the switches, and canefficiently output the power generated by the solar panels 20.

Second Embodiment

FIG. 15 is a diagram illustrating an exemplary functional configurationof a controller 60 according to a second embodiment. The controller 60according to the second embodiment further includes an efficiencydeterminer 120.

The efficiency determiner 120 determines whether the solar panel 20 isin a general efficiency state or in a low efficiency state. In thegeneral efficiency state the solar panel 20 exhibits given powergeneration efficiency. In the low efficiency state the solar panel 20exhibits lower power generation efficiency than in the generalefficiency state.

The solar panel 20 includes a plurality of clusters. All of the clustersof the solar panel 20 in normal operation receive solar light togenerate power. However, if part of the clusters is shaded, the shadedclusters do not contribute to the power generation, lowering the powergeneration efficiency of the solar panel 20 from in ordinary state.While the solar panel 20 normally receives solar light to generatepower, for example, the efficiency determiner 120 determines that thesolar panel 20 is in the general efficiency state. If the solar panel 20fails to normally receive solar light to generate power, the efficiencydeterminer 120 determines that the solar panel 20 is in the lowefficiency state.

For example, the efficiency determiner 120 acquires a value of an inputvoltage V_(IN) output from the corresponding solar panel 20. Theefficiency determiner 120 determines whether the solar panel 20 is in ageneral efficiency state or a low efficiency state from the value of theinput voltage V_(IN).

At the input voltage V_(IN) of the value exceeding a preset voltagevalue, the efficiency determiner 120 may determine that the solar panel20 is in the general efficiency state, and determine that the solarpanel 20 is in the low efficiency state when the value of the inputvoltage V_(IN) is equal to or less than the preset voltage value.

The mode controller 112 according to the second embodiment acquires aresult of the determination as to whether the solar panel 20 in questionis in the general efficiency state or in the low efficiency state, fromthe efficiency determiner 120.

When the solar panel 20 is determined to be in the general efficiencystate in the tracking mode, that is, the input voltage V_(IN) is largerthan the voltage threshold V_(T) and the input current I_(IN) is equalto or larger than the current threshold I_(T), the mode controller 112sets the power converter 30 in a tracking pass-through mode. In thetracking pass-through mode, the mode controller 112 causes the switchingcontroller 110 to stop the maximum power-point tracking. In the trackingpass-through mode, the mode controller 112 instructs the switch driver108 to control the buck-boost circuit 50 to output the input voltageV_(IN) directly as the output voltage V_(OUT) without power conversion.

When the solar panel 20 is determined to be in the low efficiency statein the tracking pass-through mode, the mode controller 112 sets thepower converter 30 in the tracking mode. In the tracking mode, the modecontroller 112 controls the switching controller 110 to execute themaximum power-point tracking.

FIG. 16 is a state transition diagram of the operation modes in thesecond embodiment. The power convertor 30 according to the secondembodiment operates in four operation modes: a stop mode, a trackingmode, a pass-through mode, and a tracking pass-through mode.

When the solar panel 20 is determined to be in the general efficiencystate in the tracking mode, the mode controller 112 controls the powerconverter 30 to transition to the tracking pass-through mode. In thetracking pass-through mode, when the solar panel 20 is determined to bein the low efficiency state, the mode controller 112 controls the powerconverter 30 to transition to the tracking mode.

Specifically, while the input voltage V_(IN) is larger than the voltagethreshold V_(T) and the input current I_(IN) is larger than the currentthreshold I_(T), and the solar panel 20 is in the general efficiencystate, such as when all of clusters normally receive solar light togenerate power, the mode controller 112 sets the power converter 30 inthe tracking pass-through mode. While the input voltage V_(IN) is largerthan the voltage threshold V_(T) and the input current I_(IN) is largerthan the current threshold I_(T), and the solar panel 20 is in the lowefficiency state, such as when part of the clusters is shaded togenerate power at lower efficiency, the mode controller 112 sets thepower converter 30 in the tracking mode.

In the tracking pass-through mode, the mode controller 112 executes thesame control as in the pass-through mode. Specifically, the modecontroller 112 gives a pass-through mode command to the switch driver108. In response to the pass-through mode command from the modecontroller 112, the switch driver 108 turns on the first switch 72,turns off the second switch 74, turns on the third switch 76, and turnsoff the fourth switch 78. In this manner, the buck-boost circuit 50 canoutput the input voltage V_(IN) directly as the output voltage V_(OUT)without power conversion.

In the present embodiment, the power conditioner 24 executes maximumpower-point tracking control over the whole solar panels 20. Meanwhile,the power convertor 30 executes maximum power-point tracking controlover one solar panel 20. Without being shaded, the solar panel 20 canoperate in the general efficiency state at the maximum power point bythe maximum power-point tracking control by the power conditioner 24under no control of the power convertor 30. While being shaded, however,the solar panel 20 cannot operate at the maximum power point unlesscontrolled by the power convertor 30. The other solar panels 20 in thesame string cannot operate at the maximum power point, either.

In the general efficiency state of the corresponding solar panel 20, thepower convertor 30 according to the present embodiment outputs the inputvoltage V_(IN) directly as the output voltage V_(OUT) without powerconversion. Thus, in the general efficiency state of the solar panel 20,the power convertor 30 can avoid unnecessary power consumption andcontrol the corresponding solar panel 20 to operate at the maximum powerpoint.

The power convertor 30 can execute maximum power-point tracking controlover the corresponding solar panel 20 operating in the low efficiencystate. Thus, the power convertor 30 can operate the corresponding solarpanel 20 at the maximum power point in the low efficiency state. In thismanner, the power convertor 30 according to the present embodiment canefficiently operate the solar panel 20 with less power consumption.

FIG. 17 is a state transition diagram of the operation modes in amodification of the second embodiment. As illustrated in FIG. 17, thepower convertor 30 according to the present embodiment may nottransition to the pass-through mode if the input current I_(IN) is thecurrent threshold I_(T) or less.

In such a configuration, as long as the solar panel 20 operates in thegeneral efficiency state, the power convertor 30 can directly output theinput voltage V_(IN) as the output voltage V_(OUT) without powerconversion. Further, the power convertor 30 can execute maximumpower-point tracking control over the corresponding solar panel 20operating in the low efficiency state. That is, the power convertor 30can efficiently operate the solar panel 20 with less power consumption.

Third Embodiment

FIG. 18 is a diagram illustrating a variation in input voltage V_(IN)during scanning in a third embodiment. A power convertor 30 according tothe third embodiment executes scanning as maximum power-point tracking.

In scanning, the switching controller 110 causes the buck-boost circuit50 to temporarily stop power conversion and opens the output end of thebuck-boost circuit 50. Subsequently, the switching controller 110 variesa value of an input voltage V_(IN) from the solar panel 20 in a givenrange to determine a value of the input voltage V_(IN) at which maximumpower is generated.

In the present embodiment, the switching controller 110 varies the inputvoltage V_(IN) in the range from a preset lower-limit value. Thelower-limit value refers to a voltage at which the power supply 62 canoperate the controller 60 including the switch driver 108.

Subsequently, the switching controller 110 calculates an amount ofcontrol over the power conversion so that the input voltage V_(IN) fromthe solar panel 20 turns to the determined voltage value. In the presentembodiment, the switching controller 110 calculates a target conversionratio at which the input voltage V_(IN) from the solar panel 20 turns tothe determined voltage value. The control amount is not limited to thetarget conversion ratio, and may be another amount. For example, thecontrol amount may be a duty ratio of the switching between the firstswitch 72 and the second switch 74 or between the third switch 76 andthe fourth switch 78.

The switching controller 110 resumes power conversion with thedetermined control amount. In the present embodiment, the switchingcontroller 110 performs switching of the buck-boost circuit 50 at thetarget conversion ratio so that the solar panel 20 generates the inputvoltage V_(IN) of the determined value. Through such operation, theswitching controller 110 can operate the solar panel 20 at a maximumpower point unless the situation changes.

FIG. 19 is a diagram illustrating the states of the switches in thebuck-boost circuit 50 in the case of varying a value of an input voltageV_(IN) from the solar panel 20 in a given range.

To vary the value of the input voltage V_(IN) from the solar panel 20,the switching controller 110 performs switching as illustrated in FIG.19. That is, the switching controller 110 causes the buck-boost circuit50 to stop outputting the output voltage V_(OUT). More specifically, theswitching controller 110 turns off the third switch 76 and turns off thefourth switch 78 to open the path between the positive output terminal46 and the negative output terminal 48.

The switching controller 110 repeatedly turns on and off the pathbetween the positive input terminal 42 and the negative input terminal44 of the buck-boost circuit 50 while changing the duty factor, therebychanging the value of the input voltage V_(IN).

To set the input voltage V_(IN) from the solar panel 20 to a targetvalue, for example, the switching controller 110 fixedly turns on thesecond switch 74 and switches on and off the first switch 72 at a dutyfactor corresponding to the target voltage value, i.e., repeatedly turnsit on and off in a given cycle. To decrease the value of the inputvoltage V_(IN), the switching controller 110 increases the duty factor,that is, elongates the on-period of the first switch 72. To increase thevalue of the input voltage V_(IN), the switching controller 110decreases the duty factor, that is, shortens the on-period of the firstswitch 72.

The switching controller 110 varies the input voltage V_(IN) from thesolar panel 20 in a range over the lower-limit value. Thus, theswitching controller 110 varies the duty factor in a range from a presetvalue.

In the present embodiment, the switching controller 110 switches thefirst switch 72 while the second switch 74 is fixedly ON. Alternatively,the switching controller 110 may switch the second switch 74 while thefirst switch 72 is fixedly ON.

FIG. 20 is a flowchart illustrating the processing of the switchingcontroller 110 according to the third embodiment.

In the tracking mode, the switching controller 110 regularly executesthe processing illustrated in FIG. 20. For example, the switchingcontroller 110 executes the processing illustrated in FIG. 20 in unit ofgiven time or at timing at which a given event occurs.

First, at S241, the switching controller 110 acquires a value of anoutput voltage V_(OUT) from the buck-boost circuit 50.

Subsequently, at S242, the switching controller 110 causes thebuck-boost circuit 50 to stop converting power, and opens the output endof the buck-boost circuit 50. Specifically, the switching controller 110turns off the first switch 72, turns on the second switch 74, turns offthe third switch 76, and turns off the fourth switch 78. In this manner,the switching controller 110 can increase the value of the input voltageV_(IN) to a maximum, placing the path between the positive outputterminal 46 and the negative output terminal 48 in open state.

At S243, the switching controller 110 sets the duty factor to 0%. AtS244, the switching controller 110 starts switching the first switch 72while the second switch 74 is fixedly ON. The switching controller 110switches on and off the first switch 72 at the set duty factor.Immediately after the start of this processing, the first switch 72 isOFF due to the set duty factor of 0%.

At S245, the switching controller 110 acquires a value of the inputvoltage V_(IN). At S246, the switching controller 110 acquires a powervalue. At S247, the switching controller 110 stores the value of theinput voltage V_(IN) and the power value in association with each other.

Subsequently, at S248, the switching controller 110 determines whetherthe input voltage V_(IN) is equal to or lower than a preset lower-limitvalue. When the input voltage V_(IN) is not equal to or lower than thelower-limit value (No at S248), the switching controller 110 proceeds toS249. At S249, the switching controller 110 increases the duty factor bya given amount, returns to S245, and repeats the processing from S245.

When the input voltage V_(IN) is equal to or lower than the lower-limitvalue (Yes at S248), the switching controller 110 proceeds to S250. AtS250, the switching controller 110 acquires a value of the input voltageV_(IN). Subsequently, at S251, the switching controller 110 acquires apower value. At S252, the switching controller 110 stores the value ofthe input voltage V_(IN) and the power value in association with eachother.

At S253, the switching controller 110 determines whether the duty factoris 0%. With the duty factor being other than 0% (No at S253), theswitching controller 110 proceeds to S254. At S254, the switchingcontroller 110 decreases the duty factor by a given amount, returns toS250, and repeats the processing from S250.

With the duty factor being 0% (Yes at S253), the switching controller110 proceeds to S255. At S255, the switching controller 110 stopsswitching the first switch 72 while the duty factor is 0%. Specifically,the switching controller 110 turns off the first switch 72.

Subsequently, at S256, the switching controller 110 specifies themaximum power value from the stored power values. At S257, the switchingcontroller 110 determines, as a target voltage value, a value of aninput voltage V_(IN) stored in association with the specified maximumpower value.

At S258, the switching controller 110 calculates, from the targetvoltage value, a target conversion ratio at which the solar panel 20 cangenerate maximum DC power. For example, the switching controller 110calculates a ratio of the value of the output voltage V_(OUT) acquiredat S241 to the determined target voltage value, and sets the calculatedratio as a target conversion ratio.

At S259, the switching controller 110 controls the buck-boost circuit 50to start power conversion at the calculated target conversion ratio.

FIG. 21 is a diagram illustrating variations in input voltage V_(IN) andinput current I_(IN) during the procedure in FIG. 20.

Through the procedure of the switching controller 110 illustrated inFIG. 20, the input voltage V_(IN) and the input current I_(IN) vary asillustrated in FIG. 21. Specifically, in the initial stage of the dutyfactor being 0%, the input voltage V_(IN) exhibits the maximum value andthe input current I_(IN) exhibits zero. Along with a gradual increase inthe duty factor from 0%, the input voltage V_(IN) gradually decreasesand the input current I_(IN) gradually increases. The input voltageV_(IN) no longer decreases after reaching the lower-limit value.

Thereafter, along with a gradual decrease in the duty factor, the inputvoltage V_(IN) gradually increases and the input current I_(IN)gradually decreases. The processing completes when the input currentI_(IN) falls to zero.

After the input voltage V_(IN) falls to the lower-limit value, theswitching controller 110 does not immediately set the duty factor to 0%but gradually decreases the duty factor to 0%. Thereby, the switchingcontroller 110 can protect the circuitry without causing backelectromotive force due to inductance components of wiring impedance.

In the present embodiment, the power convertor 30 is driven by the powergenerated by the solar panel 20. Consequently, the power convertor 30 isoperable with no receipt of power from outside.

In the maximum power-point tracking by scanning, however, setting thevalue of the input voltage V_(IN) generated from the solar panel 20 tothe minimum value (for example, zero) may cause the power convertor 30to receive no power from the solar panel 20 and stop operating. In viewof this, through the maximum power-point tracking by scanning, the powerconvertor 30 according to the present embodiment controls the inputvoltage V_(IN) from the solar panel 20 not to fall below the presetlower-limit value. Consequently, the power convertor 30 according to thepresent embodiment can reliably determine the maximum power pointwithout stopping its operation in the maximum power-point tracking byscanning.

Fourth Embodiment

FIG. 22 is a diagram illustrating an exemplary functional configurationof a controller 60 according to a fourth embodiment. The controller 60according to the fourth embodiment further includes an efficiencychanging event detector 130.

The controller 60 in FIG. 22 has the functional configuration includingthe efficiency changing event detector 130 in addition to the elementsof the controller 60 of the first embodiment illustrated in FIG. 4.However, the efficiency changing event detector 130 may be added to thefunctional configuration of the controller 60 of the second embodimentillustrated in FIG. 15.

The efficiency changing event detector 130 detects occurrence ornon-occurrence of an efficiency changing event that the power generationefficiency of the solar panel 20 varies by a preset value or more.

For example, the solar panel 20 including three clusters generates powerat 100% efficiency when all of the three clusters normally receive solarto generate power. However, if one of the three clusters is shadowed,for example, the solar panel 20 generates power at 66% efficiency. Iftwo of the three clusters are shadowed, the solar panel 20 generatespower at 33% efficiency.

In such cases, for example, the efficiency changing event detector 130detects a change in power generation efficiency of the solar panel 20from 100% to 66%, and a change in power generation efficiency of thesolar panel 20 from 66% to 33%. The efficiency changing event detector130 may detect a change in power generation efficiency of the solarpanel 20 from 33% to 66%, and a change in power generation efficiency ofthe solar panel 20 from 66% to 100%.

The efficiency changing event detector 130 may determine whether thepower generation efficiency has changed by a preset value or more, fromthe value of the input voltage V_(IN), for example. The efficiencychanging event detector 130 may determine that the power generationefficiency has changed by a preset value or more, when the value of theinput voltage V_(IN) has changed by a preset value or more.

The switching controller 110 can execute two types of maximumpower-point tracking control, i.e., by scanning and by hill climbing.The switching controller 110 switches the control between by scanningand by hill climbing in response to an efficiency changing event.

FIG. 23 is a state transition diagram of the switching controller 110according to the fourth embodiment. The switching controller 110switches the control by scanning and the control by hill climbing at thetiming illustrated in FIG. 23.

During no occurrence of an efficiency changing event, the switchingcontroller 110 executes maximum power-point tracking by hill climbing.If an efficiency changing event occurs during the maximum power-pointtracking by hill climbing, the switching controller 110 terminates themaximum power-point tracking by hill climbing, and executes maximumpower-point tracking by scanning.

In the control by scanning, the switching controller 110 determines avalue of an input voltage V_(IN) at which the solar panel 20 generatesmaximum DC power. The switching controller 110 further determines acontrol amount for the power conversion such that the solar panel 20generates maximum DC power.

In the present embodiment, the switching controller 110 determines, asthe control amount, a target conversion ratio at which the solar panel20 generates maximum DC power. The control amount is not limited to thetarget conversion ratio, and may be another amount. For example, thecontrol amount may be a duty ratio for switching on and off the firstswitch 72 and the second switch 74 or a duty ratio for switching on andoff the third switch 76 or the fourth switch 78.

After the maximum power-point tracking by scanning, the switchingcontroller 110 takes over the control amount of power conversion forgenerating the maximum DC power, and starts maximum power-point trackingby hill climbing. In the present embodiment, after the maximumpower-point tracking by scanning, the switching controller 110 startsmaximum power-point tracking by hill climbing, taking over a targetconversion ratio.

FIG. 24 is a diagram illustrating an exemplary variation in acharacteristic curve representing generated power with respect togenerated voltage when an efficiency changing event has occurred. Thesolar panel 20, while operating in the general efficiency state such asat power generation efficiency of 100%, exhibits a characteristic curvewith one peak point. However, with a decrease in the power generationefficiency caused by partial shading of the solar panel 20, for example,the solar panel 20 may exhibit a characteristic curve with two or morepeak points.

Thus, upon occurrence of an efficiency changing event, continuation ofhill climbing may cause the power convertor 30 to increase or decreasein the target conversion ratio such that the voltage value reciprocatesaround a peak point other than the maximum power point.

However, in response to occurrence of an efficiency changing event, thepower convertor 30 according to the present embodiment detects a maximumpower point through the maximum power-point tracking by scanning.Thereby, the power convertor 30 can detect a maximum power point fromthe characteristic curve with two or more peak points representinggenerated power with respect to generated voltage. Further, the powerconvertor 30 takes over a control amount (for example, target conversionratio) for the maximum power-point tracking by scanning, to executemaximum power-point tracking by hill climbing. Thus, the power convertor30 can avoid increasing or decreasing in the control amount (forexample, target conversion ratio) such that the voltage valuereciprocates around a peak point other than the maximum power point.Consequently, the power convertor 30 can operate the solar panel 20 atthe maximum power point irrespective of decrease in the power generationefficiency.

According to an additional exemplary embodiment, a power convertorincludes a buck-boost circuit to be applied with an input voltage toconvert the input voltage into an output voltage for output, the inputvoltage being generated by a power generation module that generatesdirect-current power; a switching controller that executes maximumpower-point tracking to control power conversion of the buck-boostcircuit such that the power generation module generates maximumdirect-current power; an efficiency determiner that determines whetherthe power generation module is in a first state or in a second state,the first state in which the power generation module normally generatespower, the second state in which at least part of the power generationmodule does not normally generate power and lowers in power generationefficiency from the first state; and a mode controller, wherein the modecontroller is configured to cause, in the first state, the switchingcontroller to stop the maximum power-point tracking, and the buck-boostcircuit to output the input voltage as the output voltage without thepower conversion; and cause, in the second state, the switchingcontroller to execute the maximum power-point tracking.

According to an additional exemplary embodiment, a power generationsystem includes a plurality of power generation modules connected inseries, each of which generates direct-current power; and at least onepower convertor corresponding to at least one of the power generationmodules, wherein the power convertor includes a buck-boost circuit to beapplied with an input voltage to convert the input voltage into anoutput voltage for output, the input voltage being generated by the atleast one of the power generation modules; a switching controller thatexecutes maximum power-point tracking to control power conversion of thebuck-boost circuit such that the at least one of the power generationmodules generates maximum direct-current power; an efficiency determinerthat determines whether the power generation module is in a first stateor in a second state, the first state in which the power generationmodule exerts given power generation efficiency, the second state inwhich the power generation module exerts lower power generationefficiency than in the first state; and a mode controller, wherein themode controller is configured to cause, in the first state, theswitching controller to stop the maximum power-point tracking, and thebuck-boost circuit to output the input voltage as the output voltagewithout the power conversion; and cause, in the second state, theswitching controller to execute the maximum power-point tracking.

According to an additional exemplary embodiment, a power generationcontrol method is for controlling a power generation system including aplurality of power generation modules connected in series. Each of thepower generation modules generates direct-current power, and at leastone of the power generation modules is subjected to power conversion ofa power convertor. The power convertor includes a buck-boost circuit tobe applied with an input voltage to convert the input voltage into anoutput voltage for output, the input voltage being generated by the atleast one of the power generation modules; and a switching controllerthat executes maximum power-point tracking to control power conversionof the buck-boost circuit such that the at least one of the powergeneration modules generates maximum direct-current power. The methodincludes causing, in a first state, the switching controller to stop themaximum power-point tracking, and the buck-boost circuit to output theinput voltage as the output voltage without the power conversion, thefirst state in which the at least one of the power generation modulesexerts given power generation efficiency; and causing, in a secondstate, the switching controller to execute the maximum power-pointtracking, the second state in which the at least one of the powergeneration modules exerts lower power generation efficiency than in thefirst state.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. A power convertor, comprising: a buck-boostcircuit to be applied with an input voltage to convert the input voltageinto an output voltage for output, the input voltage being generated bya power generation module that generates direct-current power, thebuck-boost circuit including a switch; a switching controller thatexecutes maximum power-point tracking by switching on and off the switchrepeatedly to control power conversion of the buck-boost circuit suchthat the power generation module generates maximum direct-current power;and a mode controller, wherein the mode controller is configured to:when an input current from the power generation module is larger than apreset current threshold, cause the switching controller to execute themaximum power-point tracking; and when the input current is equal to orless than the current threshold, cause the switching controller to stopthe maximum power-point tracking by stopping switching on and off theswitch repeatedly to cause the buck-boost circuit to output the inputvoltage as the output voltage without the power conversion, wherein themode controller causes the switching controller to stop the maximumpower-point tracking by stopping switching on and off the switchrepeatedly to cause the buck-boost circuit to output the input voltageas the output voltage without the power conversion, in response to theinput current decreasing to be equal to or less than the currentthreshold after exceeding the current threshold.
 2. The power convertoraccording to claim 1, wherein when the input voltage is equal to or lessthan a preset voltage threshold, the mode controller causes theswitching controller to stop the maximum power-point tracking and thebuck-boost circuit to stop outputting the output voltage irrespective ofthe input current.
 3. The power convertor according to claim 2, whereinthe buck-boost circuit serves as a chopper buck-boost circuit thatoutputs the input voltage as the output voltage without powerconversion.
 4. The power convertor according to claim 3, furthercomprising: a positive input terminal and a negative input terminal tobe applied with the input voltage generated from the power generationmodule; and a positive output terminal and a negative output terminalthat output the output voltage, wherein the buck-boost circuit furthercomprises: an inductor; and a capacitor connected in-between thepositive output terminal and the negative output terminal, and theswitch includes: a first switch for turning on and off a path betweenthe positive input terminal and a first terminal of the inductor; asecond switch for turning on and off a path between the negative inputterminal and the first terminal of the inductor; a third switch forturning on and off a path between a second terminal of the inductor andthe positive output terminal; and a fourth switch for turning on and offa path between the second terminal of the inductor and the negativeoutput terminal.
 5. The power convertor according to claim 4, furthercomprising a switch driver that drives the first switch, the secondswitch, the third switch, and the fourth switch, wherein the modecontroller gives a pass-through mode command to the switch driver whenthe input current is equal to or less than the current threshold, and inresponse to the pass-through mode command, the switch driver turns onthe first switch, turns off the second switch, turns on the thirdswitch, and turns off the fourth switch.
 6. The power convertoraccording to claim 5, wherein the switch driver is driven by thedirect-current power generated by the power generation module.
 7. Thepower convertor according to claim 4, wherein in the maximum power-pointtracking, the switching controller controls the power conversion of thebuck-boost circuit in accordance with a target conversion ratio, theratio representing a target ratio of a voltage value of the outputvoltage with respect to a voltage value of the input voltage.
 8. Thepower convertor according to claim 7, wherein when the target conversionratio is less than 100%, the switching controller turns on the thirdswitch and turns off the fourth switch, and complementally switches onand off the first switch and the second switch, and when the targetconversion ratio is larger than 100%, the switching controller turns onthe first switch and turns off the second switch, and complementallyswitches on and off the third switch and the fourth switch.
 9. The powerconvertor according to claim 8, wherein when the target conversion ratiois less than 100%, the switching controller complementally switches onand off the first switch and the second switch such that an on-period ofthe first switch with respect to a switching cycle is set to aproportion corresponding to the target conversion ratio, and when thetarget conversion ratio is larger than 100%, the switching controllercomplementally switches on and off the third switch and the fourthswitch such that an on-period of the third switch with respect to theswitching cycle is set to a proportion, the proportion corresponding toa value found by dividing, by the target conversion ratio, a value beinga resultant of subtracting 100% from the target conversion ratio. 10.The power convertor according to claim 7, wherein as the maximumpower-point tracking, the switching controller: further increases thetarget conversion ratio in response to an increase in the direct-currentpower caused by an increase in the target conversion ratio; decreasesthe target conversion ratio in response to a decrease in thedirect-current power caused by an increase in the target conversionratio; further decreases the target conversion ratio in response to anincrease in the direct-current power caused by a decrease in the targetconversion ratio; and increases the target conversion ratio in responseto a decrease in the direct-current power caused by a decrease in thetarget conversion ratio.
 11. The power convertor according to claim 7,wherein as the maximum power-point tracking, the switching controller:causes the buck-boost circuit to stop outputting the output voltage, andvaries the voltage value of the input voltage in a given range byswitching between the positive input terminal and the negative inputterminal while varying a duty factor; specifies, from among voltagevalues of the input voltage in the given range, a voltage value at whichthe power generation module generates the maximum direct-current power;and causes the buck-boost circuit to convert power at the targetconversion ratio so that the input voltage turns to the specifiedvoltage value.
 12. The power convertor according to claim 1, wherein thepower generation module includes a solar panel.
 13. A power generationsystem, comprising: a plurality of power generation modules connected inseries, each of which generates direct-current power; and at least onepower convertor corresponding to at least one of the power generationmodules, wherein each power convertor comprises: a buck-boost circuit tobe applied with an input voltage to convert the input voltage into anoutput voltage for output, the input voltage being generated by the atleast one of the power generation modules, the buck-boost circuitincluding a switch; a switching controller that executes maximumpower-point tracking by switching on and off the switch repeatedly tocontrol power conversion of the buck-boost circuit such that the atleast one of the power generation modules generates maximumdirect-current power; and a mode controller, wherein the mode controlleris configured to: cause the switching controller to execute the maximumpower-point tracking when an input current from the at least one of thepower generation module is larger than a preset current threshold; andwhen the input current is equal to or less than the current threshold,cause the switching controller to stop the maximum power-point trackingby stopping switching on and off the switch repeatedly to cause thebuck-boost circuit to output the input voltage as the output voltagewithout the power conversion, wherein in each power converter, the modecontroller causes the switching controller to stop the maximumpower-point tracking by stopping switching on and off the switchrepeatedly to cause the buck-boost circuit to output the input voltageas the output voltage without the power conversion, in response to theinput current decreasing to be equal to or less than the currentthreshold after exceeding the current threshold.
 14. A power generationcontrol method for controlling a power generation system comprising aplurality of power generation modules connected in series, each of thepower generation modules generating direct-current power, at least oneof the power generation modules being subjected to power conversion of apower convertor, the power convertor comprising a buck-boost circuit tobe applied with an input voltage to convert the input voltage into anoutput voltage for output, the input voltage being generated by the atleast one of the power generation modules, the buck-boost circuitincluding a switch; and a switching controller that executes maximumpower-point tracking by switching on and off the switch repeatedly tocontrol power conversion of the buck-boost circuit such that the atleast one of the power generation modules generates maximumdirect-current power, the method comprising: causing the switchingcontroller to execute the maximum power-point tracking when an inputcurrent from the at least one of the power generation modules is largerthan a preset current threshold; when the input current is equal to orless than the current threshold, causing the switching controller tostop the maximum power-point tracking by stopping switching on and offthe switch repeatedly to cause the buck-boost circuit to output theinput voltage as the output voltage without the power conversion; and inresponse to the input current decreasing to be equal to or less than thecurrent threshold after exceeding the current threshold, causing theswitching controller to stop the maximum power-point tracking bystopping switching on and off the switch repeatedly to cause thebuck-boost circuit to output the input voltage as the output voltagewithout the power conversion.